For several decades the fuels industry has tried to improve the economics of olefins oligomerization as a means to make gasoline and diesel. However, the yield and overall economics of alternative processes, especially isoparaffin-olefin alkylation, have been too challenging mainly because alkylation also upgrades iso-C4 hydrocarbons in available refinery feedstocks and produces a higher quality product.
Similarly, researchers have tried to make processes for upgrading light paraffins more economical. However, although there is an over-supply of light paraffins in the market and margins are attractive, the high cost, mainly due to the endothermic nature of the reaction, as well as low yield have prevented much commercial application of this important chemistry. In fact, light olefins, and particularly light paraffins upgrading, is one of the highest margin improvement upgrading technologies per barrel of feed. Margin uplift can be as high $15-30/BBL. However, current technologies have failed to capture much of this potential.
There is therefore a need in the art of fuels refining for an economically viable process to incorporate paraffins into the feedstocks used in the oligomerization of olefins for commercial gasoline and diesel production. There is also interest in minimizing the benzene content of the product.
U.S. Pat. No. 3,845,150 discloses a process for the aromatization of hydrocarbon streams over a ZSM-5 catalyst in which the heat input requirements normally associated with the aromatization of saturated hydrocarbons is substantially eliminated by combining the saturated hydrocarbon feed with an unsaturated hydrocarbon feed such that the combined feed contains 20 to 65 weight % of reactive saturates and 20 to 50 weight % of olefins. The process is conducted at a temperature of 650 to 1500° F. (343 to 816° C.) and a weight hourly space velocity of 0.1 to 15 in the absence of added hydrogen. However, the process suffers from the disadvantage that the conversion conditions are necessarily a compromise between the optimal conditions for upgrading the reactive saturates and optimal conditions for upgrading the olefins. As a result, the yields of the desired products are low.
U.S. Pat. No. 4,788,364, hereby incorporated by reference, discloses an improved two-step process for the conversion of lower molecular weight paraffins, the process comprising contacting in a first step a C2-C10 alkane-rich feedstock with a siliceous zeolite catalyst in a primary fluidized bed reaction zone under high temperature dehydrogenation conditions to obtain an intermediate product comprising oligomerizable olefinic hydrocarbons and aromatics; and then contacting in a second step the intermediate product with a siliceous zeolite catalyst in a secondary fluidized bed reaction zone under low temperature oligomerization conditions to obtain a final product comprising gasoline boiling range aliphatic and aromatic hydrocarbons. Thermal balance is maintained by employing a heat exchange line containing steam or preheated feedstock. However, the addition of steam can deactivate the zeolite catalyst while preheating the feed can lead to cracking and hence fouling of the feed preheater.